Persson, Åsa Helen

I dette projekt blev der udført højtemperatur oxidationseksperimenter af slurry-belagte ferrit legeringer i atmosfærer sammenlignelig med atmosfæren ved katoden i en SOFC. Baseret på resultaterne opnået under studiet er der formuleret mulige interaktionsmekanismer mellem slurry-belægninger og de voksende oxidlag på overfladen af legeringsprøverne. Disse mekanismer er et skridt mod en dybere forståelse af, hvordan man skal designe en kombination af belægning/legering med tillfredstillende egenskaber til en SOFC-stak. Tillfredstillende egenskaber indebærer en lav væksthastighed af oxidlaget på legeringsoverfladen og lavt kromindhold samt en lav elektrisk modstand i dette oxidlag. Slurry-belagte ferrit legeringsprøver blev oxideret under lang tid i luft med 1% vand ved 900˚C for at måle oxidationshastigheden af de belagte prøver. De ferrit-legeringer, som var inkluderet i studiet, var Crofer 22APU og Sandvik 1C44Mo20. Nogle supplerende eksperimenter blev udført på forskellige Sandvik legeringer. Slurry-belægninger bestod af perovskit, spinel, korundum, og rutile oxider og de blev pålagt ved sprøjtning både som enkeltlagsbelægninger og som dobbeltlagsbelægninger i de tilfælde, hvor de blev kombineret. Tværsnit af de oxiderede prøver blev analyseret med skanning elektron mikroskopi, SEM, og energi dispersiv spektroskopi, EDS. Disse analyser gav informationer om oxidlagenes mikrostruktur og kemiske sammensætning. Baseret på målingerne af oxidationshastigheden af de enkeltlags belagte legeringsprøver kunne tre mulige interaktionsmekanismer mellem belægning og det voksende oxidlag formuleres: A Overfladedækkende belægning B Overfladedækkende belægning med kemisk reaktion med det voksende oxidlag C Inkorporering af belægningspartikler I interaktionsmekanisme A skubbes belægningspartiklerne foran det udad voksende oxidlag. Belægninger med denne type af interaktionsmekanisme giver hovedsaglig en geometrisk beskyttelse mod oxidation gennem blokering af ilt tilgang ved overfladen af oxidlaget. Beskyttelseeffekten forsvinder, når oxidlaget vokser tykkere end diameteren af beskyttelsepartiklerne. Interaktionsmekansime B medfører en kemisk reaktion mellem det voksende oxidlag og belægningspartiklerne, samtidigt med at belægningspartiklerne skubbes foran det voksende oxidlag. Den kemiske reaktion fører til en forandring i oxid sammensætningen. I den tredje og sidste interaktionsmekanisme, C, inkorporeres belægsningspartiklerne i det voksende oxidlag. De inkorporerede belægsningspartikler opbygger en geometrisk beskyttelse mod oxidation og denne beskyttelse skulle ikke miste sin effekt, når oxidlaget har vokset sig tykkere end diameteren af belæsgningspartiklerne. De to enkeltlagsbelægninger bestående af (La0.85Sr0.15)MnO3 + 10% overskydende Mn, LSM, og (La0.85Sr0.15)CoO3 + 10% Co3O4, LSC, var succesfuld m.h.t. at mindske iv oxidationshastigheden, krom indholdet og den elektriske modstand i det voksende oxidlag på Crofer 22APU prøverne. Den positive effekt af LSM og LSC belægningerne blev forøget endog mere, når de var i kombination med et indre Co3O4-belægningslag. For Sandvik 1C44Mo20 prøverne belagt med Co3O4 + LSM blev der kun observeret en lille mindskning i oxidationshastigheden i forhold til den LSM belagte Sandvik 1C44Mo20 prøve. For Sandvik 1C44Mo20 prøverne belagt med Co3O4 + LSC blev der observeret en drastisk øgning af oxidationshastigheden i forhold til Sandvik 1C44Mo20 prøver belagt med et enkelt lag af LSC. En signifikant mængde krom blev tillige observeret i oxidlaget på de Co3O4 + LSC belagte Sandvik 1C44Mo20 prøver. Forskellen i overskuddet af mangan for Crofer 22APU og Sandvik 1C44Mo20 prøverne er blevet foreslået som en mulig forklaring på, at de kobolt-rige belægningerne giver en positiv effekt på Crofer 22APU legeringsprøverne og deres manglende effekt på Sandvik 1C44Mo20 legeringsprøverne. Det indikerede at der var brug for en vis mængde mangan for at stabilisere oxidlagets vækst på legeringsprøver belagte med en kobolt-rig belægning. Supplerende oxidationseksperimenter for Co3O4 + LSC belagte Sandvik legeringsprøver med varierende mangan indhold understøttede denne hypotese. Baseret på mikrostrukturen af de dannede oxidlag på alle Co3O4 + LSC belagde Fe-22Cr legeringsprøver i dette studie og deres vægtøgning, forekommer det, at der er brug for et optimalt mangan indhold i legeringen på omkring 0.3-0.5 vægt. % for at kunne udnytte de positive effekter af kobolt-rige belægninger.In this project, high temperature oxidation experiments of slurry coated ferritic alloys in atmospheres similar to the atmosphere found at the cathode in an SOFC were conducted. From the observations possible interaction mechanisms between the slurry coatings and the growing oxide scale on the alloy surface were formulated. These mechanisms are a step towards deeper knowledge of how to design a coating/alloy combination with satisfactory performance in an SOFC-stack. A satisfactory performance entails a low growth rate of the forming oxide scale on the alloy surface and a low chromium content and resistance in this oxide scale. Slurry coated ferritic alloy samples were oxidized long term in air containing 1% water at 900˚C to measure the oxidation rate of the coated samples. The ferritic alloys included in the study were Crofer 22APU and Sandvik 1C44Mo20. Some complementary experiments were also performed on extra Sandvik alloys. The slurry coatings consisted of perovskite, spinel, corundum, and rutile oxides and they were both applied as single layer coatings and as dual layer coatings. Cross-sections of the oxidized samples were analyzed with scanning electron microscopy, SEM, and energy dispersive spectrometry, EDS. From these analyses information about the microstructures and the compositions of the grown oxide scales were collected. Based on the oxidation rate measurements for the single layer coated alloy samples three possible interaction mechanisms between the coating and the growing oxide scale were formulated:A Surface covering coatingB Surface covering coatings with chemical reaction with the forming oxideC Incorporation of coating particlesIn the interaction mechanism A the coating particles are pushed ahead of the outwardly growing oxide scale. Coatings with this interaction mechanism mainly give a geometrical protection against oxidation by blocking oxygen access at the surface of the oxide scale. The protecting effect is gradually reduced as the oxide scale grows thicker than the diameter of the coating particles. Interaction mechanism B entails a chemical reaction between the growing oxide scale and the coating particles at the same time as these are pushed ahead of the growing oxide scale. The chemical reaction leads to a change in the oxide composition. In the third and last interaction mechanism, C, the coating particles are incorporated into the growing oxide scale. The incorporated coating particles create a geometrical protection against oxidation that should not loose their effect after the oxide scale has grown thicker than the diameter of the coating particles.The two single layer coatings consisting of (La0.85Sr0.15)MnO3 + 10% excess Mn, LSM, and (La0.85Sr0.15)CoO3 + 10% Co3O4, LSC, coatings were found to be relatively successful in decreasing the oxidation rate, the chromium content in the outermost part of the dense scale, and the electrical resistance in the growing oxide scales when applied onto Crofer 22APU. But, the positive effects of the LSM and LSC coatings were significant enhanced when they were combined with an inner Co3O4 coating layer. However, for the Sandvik 1C44Mo20 samples the samples coated with Co3O4 + LSM only displayed a slight decrease in the oxidation rate compared to the LSM coated samples. For the Sandvik 1C44Mo20 samples coated with Co3O4 + LSC a drastic increase in the oxidation rate was observed compared to the Sandvik 1C44Mo20 samples coated with LSC. A significant amount of chromium in the oxide scale on the Co3O4 + LSC coated Sandvik 1C44Mo20 samples was furthermore detected. The cobalt rich coatings’ success on Crofer 22APU alloy samples and their failure on Sandvik 1C44Mo20 samples are believed to depend on the manganese access in the coating/alloy system. It appeared that a certain amount of manganese was acquired to stabilize the oxide growth on the alloy samples coated with cobalt rich coatings. This was supported by complementary oxidation of Co3O4 + LSC coated Sandvik alloys with varying manganese contents. Based on the microstructures of the formed oxide scales on all Co3O4 + LSC coated Fe-22Cr alloys samples included in this study and their observed weight gain, it was established that an optimal manganese content in the alloy on ca. 0.3-0.5 wt.% is a requirement to stabilize and utilize the positive effects of a cobalt rich coating

Online Research Database In Technology

High temperature oxidation of slurry coated interconnect alloys

http://orbit.dtu.dk/ws/files/9558090/Phd_Thesis_sa_120522.pdf

High temperature oxidation of slurry coated interconnect alloys

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Online Research Database In Technology